Process for producing thermoplastic resin granules

ABSTRACT

The present invention concerns a process for producing thermoplastic resin granules, comprising compression-molding a powdery raw material of a thermoplastic resin, which is obtained by polymerizing a plurality of materials and then drying the obtained polymer by passing the powdery raw material between two rolls arranged parallel with a minute gap therebetween at a temperature of 40° C. or higher, and crushing the obtained compression-molded product into granules having grain diameter of 10 mm or less, wherein concavities are formed all around the rolls so as to be arrayed in a direction inclined relative to axes of the rolls, the concavities each having an elliptical opening of which a major-axis diameter measures 10 mm or less.

TECHNICAL FIELD

The present invention relates to a process for producing thermoplasticresin granules to be supplied to a granulating machine or secondarymolding machine.

BACKGROUND ART

In the production process of a thermoplastic resin, a powdery rawmaterial of the thermoplastic resin, which is obtained by polymerizing aplurality of materials and then drying the obtained polymer, is cooled,is then mixed with an auxiliary material such as a plasticizer oranother resin material, and is then granulated by a screw extruder intopellets having grain diameters of approximately 10 mm or less. The thusobtained pellets are supplied to a secondary molding machine such as aninjection molding machine or blow molding machine so as to be moldedinto the shape of an end product.

The powdery raw material of a thermoplastic resin just after drying andcooling as described above has a low bulk density, and thus requiresunduly large pieces of equipment for its transportation and storage.Moreover, in a screw extruder, the low bulk density of the powdery rawmaterial incurs not only large slippage between the screw and the casingthereof but also a large compression ratio, which leads to unduly highpower consumption by the screw extruder when it is driven.

These inconveniences can be alleviated by first compression-molding thepowdery raw material into flakes by passing it between two rollsarranged parallel with a minute gap therebetween, and then crushing theobtained flakes into granules having grain diameters of 10 mm or less.

However, the thus obtained granules do not have a satisfactorily highbulk density, and thus they do not offer desired mechanical strength.

Moreover, attempting to produce granules having uniform grain sizeswithin a narrow range results in low yields.

DISCLOSURE OF THE INVENTION

A first object of the present invention is to provide thermoplasticresin granules having a satisfactorily high bulk density.

A second object of the present invention is to efficiently providethermoplastic resin granules having satisfactorily uniform grain sizes.

To achieve the above objects, according to a first aspect of the presentinvention, a process for producing thermoplastic resin granules includescompression-molding a powdery raw material of a thermoplastic resin,which is obtained by polymerizing a plurality of materials and thendrying the obtained polymer, by passing the powdery raw material betweentwo rolls arranged parallel with a minute gap therebetween at atemperature of 40° C. or higher, and crushing the obtainedcompression-molded product into granules having grain diameters of 10 mmor less.

According to a second aspect of the present invention, a process forproducing thermoplastic resin granules includes mixing a powdery rawmaterial of a thermoplastic resin, which is obtained by polymerizing aplurality of materials and then drying the obtained polymer, with anauxiliary material at a temperature of 40° C. or higher,compression-molding the powdery raw material by passing the powdery rawmaterial between two rolls arranged parallel with a minute gaptherebetween, and crushing the obtained compression-molded product intogranules having grain diameters of 10 mm or less.

According to a third aspect of the present invention, in the process forproducing thermoplastic resin granules of the first or second aspect ofthe present invention, the rolls each have a large number of identicallyshaped and identically sized concavities formed all over the outercircumferential surface thereof. Here, the concavities each have anelliptical opening of which the major-axis diameter measures 10 mm orless and is aligned with the direction of the circumference of the outercircumferential surface of the roll. Moreover, the concavities each havea curved surface of which the section on a plane parallel to thedirection of the circumference is arc-shaped. Furthermore, theconcavities are arranged in such a way that the minimum distance betweenany two adjacent concavities is 0.5 mm or less and that a concavity inone roll faces a concavity in the other roll across the minimum gapbetween the two rolls.

According to a fourth aspect of the present invention, in the processfor producing thermoplastic resin granules of the third aspect of thepresent invention, the minimum gap between the two rolls is 0.5 mm ormore and 1.0 mm or less.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a thermoplastic resin pellet productionprocess that incorporates a thermoplastic resin granule productionprocess embodying the present invention.

FIG. 2 is a block diagram of another thermoplastic resin pelletproduction process that incorporates a thermoplastic resin granuleproduction process embodying the present invention.

FIG. 3 is a sectional view, as seen from the front, of a roll pressemployed in the present invention.

FIG. 4 is a detail view of a principal portion of FIG. 3.

FIG. 5 is a different view of FIG. 4, as seen from the directionindicated by arrows 5 in FIG. 4.

FIG. 6 lists data showing the relationship between the temperature ofthe powdery raw material of polycarbonate and the bulk density of thethermoplastic resin granules obtained from that raw material.

FIG. 7 lists data showing the relationship between the temperature ofthe powdery raw material of polycarbonate and the crush strength anddensity of the compression-molded product obtained from that rawmaterial.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

FIGS. 1 and 2 are block diagrams of thermoplastic resin pelletproduction processes that incorporate a thermoplastic resin granuleproduction process embodying the present invention. FIG. 3 is asectional view, as seen from the front, of a roll press employed in thepresent invention. FIG. 4 is a detail view of a principal portion ofFIG. 3. FIG. 5 is a different view of FIG. 4, as seen from the directionindicated by arrows 5 in FIG. 4.

In the thermoplastic resin pellet production process shown in FIG. 1, apowdery raw material of a thermoplastic resin, which is obtained bypolymerizing a plurality of materials and then drying the obtainedpolymer, is compression-molded by being passed between two rollsarranged parallel with a minute gap therebetween without being cooled,specifically at a temperature of 40° C. or higher, and preferably 50° C.or higher. The obtained compression-molded product is then crushed intogranules having grain diameters of 10 mm or less. The thus obtainedgranules are mixed with an auxiliary material and then supplied to ascrew extruder so as to be granulated into pellets.

On the other hand, in the thermoplastic resin pellet production processshown in FIG. 2, a powdery raw material of a thermoplastic resin afterpolymerization and drying is mixed with an auxiliary material withoutbeing cooled, specifically at a temperature of 40° C. or higher, andpreferably 50° C. or higher, and is then compression-molded by beingpassed between two rolls arranged parallel with a minute gaptherebetween. The obtained compression-molded product is then crushedinto granules having grain diameters of 10 mm or less. The thus obtainedgranules are supplied to a screw extruder so as to be granulated intopellets.

The thermoplastic resin used here may be of any kind, for example,polyethylene, polyethylene terephthalate, polystyrene, ABS resin,metacrylate resin, polyamide, polycarbonate, polyacetal, polyphenyleneether, polyphenylene sulfide, polyether ether ketone, polysulfone,fluororesin, polybutylene terephthalate, or the like.

The auxiliary material added here may be, for example, a plasticizer,stabilizer (polyvinyl chloride stabilizer, softening stabilizer,hardening stabilizer), flame retarder, anti-oxidant (oxidationinhibitor), ultraviolet ray absorbent, colorant, antistatic agent,reinforcer (glass fiber, carbon fiber, aramid fiber, boron fiber,synthetic fiber (vinylon, polyester)), filler (for the purpose ofreinforcement, shielding, electric conduction, lubrication, absorption,anti-dripping, weather resistance, thermal expansion coefficientadjustment, improved printability or adhesiveness, extending, or other),or the like.

The obtained pellets are supplied to a secondary molding machine so asto be molded into the shape of an end product.

The secondary molding machine here may be, for example, a compressionmolding machine, calendering machine, extrusion molding machine, blowmolding machine, vacuum or compressed-air molding machine, foamingmachine, injection molding machine, or the like.

The granules may be supplied intact, i.e. without being granulated intopellets, to the secondary molding machine.

Since a thermoplastic resin becomes plastic at high temperatures, bycompression-molding a powdery raw material thereof just after drying bypassing it between rolls without cooling it, i.e. at a temperature of40° C. or higher, it is possible to obtain granules having a high bulkdensity.

FIG. 6 shows the relationship between the bulk density of the obtainedgranules and the temperature of the powdery raw material when a powderyraw material of polycarbonate, having a bulk density of 0.21 g/cm² afterpolymerization and drying, is first compression-molded into 4 mm thickflakes by being passed between two rolls under compressive force of 4 tfor every 1 cm width along the axes of the rolls, and is then crushedand sifted to obtain granules having grain diameters of 3 to 5 mm. FIG.6 shows that, the higher the temperature of the powdery raw material,the higher the bulk density of the obtained granule.

When a highly heat-resistant thermoplastic resin such as a polyphenylenesulfide resin is used, at an ordinary temperature of 25° C., it is notpossible to obtain a satisfactorily hard product under molding pressureof 1 t/cm², and thus the product does not stand the shock of pneumatictransportation and is smashed thereby. On the other hand, with thepowdery raw material at 100° C. or higher, it is possible to obtain asatisfactorily hard product. Thus, the preferable temperature of thepowdery raw material is 100° C. or higher.

FIG. 7 shows the data of the crush strength and density of the productobtained when a cylinder measuring 25 mm in diameter and 25 mm in heightis filled with a powdery raw material of a polyphenylene sulfide resinand then the resin is compression-molded under pressure of 1 t/cm²applied thereto by a piston, separately for cases where the temperatureof the powdery raw material is 25° C. and 100° C. respectively.

The rolls may have flat, smooth circumferential surfaces, or have finegrooves formed in the circumferential surfaces thereof along the axis orcircumference thereof, or have undulate circumferential surfaces. Thetwo rolls are arranged with a gap of about 2 to 5 mm therebetween. Witha highly plastic resin, however, the crushed product has irregularshapes and very rugged cut surfaces, which often leads to anunexpectedly low bulk density. Moreover, the crushed product exhibits anunduly wide grain size distribution, and thus, to obtain granules havinguniform grain sizes, the crushed product needs to be sifted so thatgranules having grain sizes larger than desired are crushed again andthat granules having grain sizes smaller than desired are circulated soas to be compression-molded between the rolls and then crushed. Thisleads to low yields.

For these reasons, when a highly plastic resin is used, as shown inFIGS. 3 to 5, compression molding is performed using briquette rolls 1and 1 that have a large number of equally shaped and equally sizedconcavities 1 a, 1 a, . . . formed all over the outer circumferentialsurfaces thereof, with the rolls arranged with a gap d of 0.5 mm or moreand 1.0 mm or less therebetween. The concavities 1 a, 1 a, . . . areeach so formed as to have an elliptical opening of which the major-axisdiameter is 10 mm or less and is aligned with the direction of thecircumference of the outer circumferential surfaces of the rolls 1, andare arranged in such a way that the concavities in one roll 1 face theconcavities in the other roll 1 across the minimum gap d (see FIG. 4)between the rolls 1 and 1.

The concavities la formed on the outer circumferential surfaces of therolls 1 and 1 help obtain the product in the form of a large number ofbriquettes connected together. This product is then crushed by acrushing machine that breaks it at the borders between briquettes, i.e.at the thin portions of the product that are formed by the roundportions 1 b of the outer circumferential surfaces of the rolls 1 wherethere are no concavities 1 a, into separate briquettes. The thusobtained briquettes are not flat, but have smooth surfaces, and thushave a higher bulk density and better fluidity than irregularly-shapedgranules obtained by crushing flakes as described earlier. This makesuniform discharge of the product from a hopper and uniform supplythereof to a screw extruder possible.

Here, since a powdery raw material of a thermoplastic resin is apt toexhibit high friction resistance against the outer circumferentialsurfaces of the rolls, the friction resistance occurring between thesurfaces of the concavities 1 a and the product at the position wherethe highest molding pressure is present (the position indicated by p inFIG. 4) tends to produce so strong shearing force as to cause cracks inand thus breakage of the product. To prevent this, the concavities 1 aare each so formed as to have a curved surface of which the section on aplane parallel to the circumference of the outer circumference surfacesof the rolls 1 is arc-shaped. This permits the product to slip on thesurfaces of the concavities 1 a, and thereby prevents generation ofunduly strong shearing force. To ensure that, when crushed, the productis broken at the borders between briquettes, the round portions 1 b needto be made as small as possible. Specifically, the concavities 1 a, 1 a,. . . are arranged in such a way that the minimum distance h between anytwo adjacent concavities 1 a and 1 a is 0.5 mm or less.

To prevent unduly strong compressive force near the round portions 1 bfrom causing cracks in briquettes, or breakage thereof that may causebroken briquettes to attach to the concavities 1 a, the gap d betweenthe rolls is made to be 0.5 mm or more. In addition, to prevent theround portions 1 b from having so large a thickness as to permitbriquettes to be broken somewhere other than at the thin portions, thegap d is made to be 1.0 mm or less.

The raw material charged into a hopper 3 through a raw material inlet 2is then supplied by a screw 4 to between the rolls 1 and 1, where theraw material is compression-molded by the rolls 1 and 1 rotating asindicated by arrows. Here, at the position p where the rolls 1 and 1have the minimum gap, the product slips on the surfaces of theconcavities 1 a, and thus it does not develop unduly strong shearingforce inside itself. Moreover, since the round portions 1 b between theconcavities 1 a and 1 a are small, the product is obtained in the formof a large number of briquettes connected together by thin portions.When crushed, the product is broken at the thin portions into separatebriquettes.

As one example, a powdery raw material of a polyphenylene sulfide resinafter polymerization and drying was compression-molded into flakes bybeing passed between a pair of rolls arranged parallel with a gap of 4mm therebetween and having flat, smooth outer circumferential surface ata high temperature of 75° C. under compressive force of 4 t for every 1cm width along the axes of the rolls. The obtained flakes were crushedand sifted to obtain irregularly-shaped granules having grain diametersof 3 to 5 mm. The bulk density of the thus obtained granules was 0.42g/cc.

As another example, the same material was compression-molded into aproduct in the form of a large number of briquettes connected togetherby being passed, at the same temperature and under compressive force of4 t/cm, between a pair of rolls arranged parallel with a gap of 1 mmtherebetween and having a large number of concavities 1 a, 1 a, . . .formed in the outer circumference surfaces thereof, with the concavitiesmeasuring 5 mm in major-axis diameter along the direction of thecircumference of the rolls, 4 mm in minor-axis diameter along thedirection of the axis of the rolls, and 1.5 mm in depth and arranged insuch a way that the minimum distance between any two adjacentconcavities 1 a and 1 a is 0.3 mm. The obtained product was crushed toobtain granules in the form of separate briquettes measuring 5 mm inlength, 4 mm in width, and 4 mm in thickness. The bulk density of thethus obtained granules was 0.56 g/cc.

In an attempt to produce granules having comparatively uniform graindiameters of 4 to 6 mm, a powdery raw material was compression-moldedinto flakes by being passed between rolls having flat, smooth outercircumferential surfaces, and then the obtained flakes were crushed andsifted so that granules having grain diameters of 4 to 6 mm werecollected, that granules having grain diameters larger than 6 mm werecrushed again, and that granules having grain diameters smaller than 4mm were circulated so as to be compression-molded again. This attemptresulted in a yield of about 30%. By contrast, in another attempt to thesame effect, the powdery raw material was compression-molded intobriquettes measuring 6 mm in length, 4.5 mm in width, and 4.5 mm inthickness by being passed between rolls having concavities, measuring 6mm in major-axis diameter and 4 mm in minor-axis diameter, formed in theouter circumferential surfaces thereof, and then the obtained briquetteswere sifted with a sieve having 4 mm meshes so as to remove small burrsbetween briquettes and fine particles that have escaped through the gapsat both sides of the rolls. This attempt resulted in a yield of 90% ormore.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced other than as specifically described.

Industrial Applicability

As described above, by a thermoplastic resin granule production processembodying the present invention, it is possible to obtain thermoplasticresin granules having a high bulk density, and thereby reduce the sizesof pieces of equipment for its transportation and storage, and inaddition reduce the power consumption by a screw extruder.

In particular, by the thermoplastic resin granule production processeven with a highly plastic thermoplastic resin, it is possible topermit, during compression molding, the product to slip on the surfacesof concavities, and thus prevent generation of unduly strong shearingforce in the product and thereby reduce development of cracks therein.Moreover, since the distances between adjacent concavities are small, itis possible to make the portions connecting briquettes together so thinas to reduce the possibility of briquettes themselves being broken whenthe product is crushed. Furthermore, it is possible to efficientlyobtain thermoplastic resin granules having uniform grain sizes.

By the thermoplastic resin granule production process it is possible toreduce the possibility of unduly strong compressive force causing cracksin briquettes, or breakage thereof that may cause broken briquettes toattach to concavities, and in addition further reduce the possibility ofbriquettes themselves being broken when the product is crushed.

What is claimed is:
 1. A process for producing thermoplastic resingranules, comprising compression-molding a powdery raw material of athermoplastic resin, which is obtained by polymerizing a plurality ofmaterials and then drying the obtained polymer, by passing the powderyraw material between two rolls arranged parallel with a minute gaptherebetween at a temperature of 40° C. or higher, and crushing theobtained compression-molded product into granules having grain diameterof 10 mm or less, wherein concavities are formed all around the rolls soas to be arrayed in a direction inclined relative to axes of the rolls,the concavities each having an elliptical opening of which a major-axisdiameter measures 10 mm or less.